Spec Miata Engine Modifications Information

A. Induction System
1. All air entering the intake tract shall pass through the fuel injection air inlet.
2. 1.6L cars may replace the stock air box with a cone style air filter assembly. The air filter element is unrestricted. No ducting or baffling of air to the air filter is permitted.
3. 1.6L cars may open and adjust, but not modify, the OEM airflow meter. For 1.6L cars, the position of the air flow meter may be moved provided it remains attached to the unmodified factory intake tube.
4. 1.8L cars must use the stock air box, but the air filter element is unrestricted.
5. 1.8L cars shall use an air restrictor. The throttle restrictor shall be placed between the throttle body and plenum. All intake air shall pass through the restrictor plate.
Restrictor plates must be the proper size as listed in the Specification Table, must be from Mazdaspeed Motorsports Development, and must not be modified.

B. Fluid hoses and clamps, oil filters, fuel filters, and belts (fan, alternator, etc.) may be substituted with others of equivalent OEM specifications.

C. Fuel System
1. Unleaded fuel filler trap door and restrictor plate in filler neck may be removed.
2. Refer to GCR Section 9.3.25 for permitted fuel specifications and for the required fuel sample acquisition port.

d. Ignition/Electrical System
1. Spark plugs and spark plug wires may be substituted.
2. Ignition timing is unrestricted within stock adjustment capability.
3. Batteries may be replaced with those of an alternate manufacturer, provided they are of similar amp-hour capacity, size, and weight and are fitted in the standard location.
Additional battery hold-down devices may be used, and are strongly recommended.

E. Exhaust System
1. The factory exhaust system beyond the OEM front down pipe may be replaced, provided:
a. Said replacement system retains the original configuration, e.g. single tube design, and the tubing diameter is a maximum of 2.25” outside diameter.
b. The pipe may end anywhere after the rear subframe. Forward of the rear subframe, the pipe must follow the original path of the OEM exhaust system.
c. No expansion chambers. A single muffler may be added.
d. The system meets all event specific sound requirements.
e. A cat replacement tube may be installed. The tube shall not exceed 17.5” in length and have an outside diameter no greater than 2.375”.
2. Original exhaust system heat shields may be removed.

F. Clutch System and Flywheel: All cars shall use either the stock OEM pressure plate appropriate model year or the ACT pressure plate (Mazdaspeed p/n: 0000-0205401-SS (1.6L cars) or 0000-0205404-AC (1.8L cars)). The unmodified pressure plate shall be bolted directly to the appropriate stock, unmodified flywheel. The 94 model year may utilize the flywheel from the 95-05 model years. Any clutch disk may be used. The minimum weight (including the pilot bearing) is 17.6 lbs for the 1.6L and 17.1 lbs for the 1.8L.

G. Lubricants may be substituted with any lubricant. Oil additives are unrestricted.

H. The engine block and/or cylinder head may be decked/milled to achieve the factory specified compression ratio for the correct model year as listed on the specification table.

I. Camshafts shall comply with the Official Camshaft Data as supplied by the SCCA Club Racing Tech Department.

J. Cast iron cylinder liners (sleeves) may be installed to restore damaged or worn cylinder bores to the original dimension.